Accumulating evidence suggests that dysregulation of glutamatergic transmission in brain regions involved in mood regulation, such as prefrontal cortex (PFC), is linked with depressive disorders. In addition, applications of N-methyl-D-aspartate receptor (NMDAR) antagonists, such as ketamine, exhibit fast-acting and long-lasting antidepressants properties. However, despite these promising findings, limitations of ketamine use as an antidepressant treatment, particularly its dissociative/psychotomimetic effects and abuse potential, highlight the need for alternative glutamatergic agents. Our long-term research goal is to search for and characterize novel glutamatergic agents that exhibit fast-acting antidepressant effects. The objective for this application is to evaluate the antidepressant potential of D- serine, an endogenous NMDAR co-agonist that acts on glutamatergic synapses. The proposed ketamine treatment mechanisms pertain to rapidly enhanced glutamate transmission and synaptogenesis, possibly through inhibition of local GABAergic interneurons and postsynaptic activation of intracellular signaling cascades in the mTOR pathway. D-serine, as an endogenous NMDAR co-agonist, also boosts glutamate transmission and synaptogenesis. Our preliminary results along with previous studies lead to our central hypothesis that activation of NMDAR co-agonist site by D-serine results in fast-acting antidepressant effects. Moreover, D-serine may work cooperatively with ketamine to lower its therapeutic threshold, diminishing the likelihood of abuse. In aim 1, we will evaluate the therapeutic potential of D-serine as a fast-acting antidepressant in preclinical models of depression. Aim 2 is designed to examine the effectiveness of combining D-serine and ketamine at their therapeutic or sub-threshold doses. Finally, we will delineate the synaptic and neural circuitry mechanisms of D-serine and ketamine antidepressant actions, alone or in combination in aim 3. Upon completion, the studies proposed above are likely to generate mechanistic information on D-serine's fast-acting antidepressant potential as well as its feasibility of reducing adverse side effects of ketamine by lowering its therapeutic threshold.